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It is possible to take a continuous set of sights, plot a curve, and establish
an approximate time for local noon.  But the greater the zenith distance from
the sun, the slower the sun will rise as it gets close to noon, and the
shallower therefore will be the curve before and after noon.  Picking the
precise moment of noon will still be guesswork.

There is another time-honored way to do this.  It has appeared from time to
time in these discussions, and you can possibly still find it in the archives
("threads").  Its beauty lies in the fact that the sun rises toward noon and
declines after noon at the same rate.  Thus, if you draw the curve on graph
paper, it will be symmetrical.

This means that if you time a sight before noon, let's say, in a sextant sight
of 64 degrees exactly, you need merely set your sextant at 64 degrees again
after you are conscious that noon has passed, since the sun is now declining
in altitude.  Wait for the sun to drop to the equator with the sun preset on
the sextant, and mark the precise time of the second sight at that altitude.
Determine the difference in time between the two sights; one half that
difference, added to the time of the first sight, will be the time of local
noon.  It's even better to take a series of equal altitude sights and
establish for each pair a time of local noon, then use an average.

Hew Schlereth's book on navigation by the noon sight uses this method, and
before him the Admiralty Manual of Navigation and many other sources.

It is true that if your boat is moving between sights, there is a component of
your altitude in taking the second sight, the one after noon, that will be
attributable to a change in your zenith distance from the sun, and this will
reflect itself in a slight difference in the time between the two sights.  But
in boats of our persuasion, the change is usually negligible, and it requires
a heavy north-south component to make much of a difference.  Schlereth's book
has a correction table for it, as does the Admiralty manual.

Mal Misuraca
Passage East
Sausalito
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